What Is Insulin, Human Isophane (NPH)? Uses, Dosage, and What You Need to Know in 2026

Insulin, human isophane — commonly known as NPH insulin — is one of the oldest and most widely used forms of insulin in the world. If you or a loved one has been prescribed Humulin N, Novolin N, or ReliOn N, this guide explains exactly what this medication is, how it works, how to use it, and what you need to know to stay safe.

What Is Insulin, Human Isophane (NPH)?

Insulin, human isophane is an intermediate-acting human insulin. The name NPH stands for Neutral Protamine Hagedorn — "neutral" for its pH of 7.0-7.5, "protamine" for the protein added to slow its absorption, and "Hagedorn" for Hans Christian Hagedorn, the Danish scientist who developed it in 1946.

It is a biosynthetic product — meaning it is made in a laboratory using recombinant DNA technology to produce human insulin that is structurally identical to the insulin made by the human pancreas. Unlike rapid-acting or long-acting insulins, NPH sits in the middle: it starts working within 1-3 hours, peaks at 4-12 hours, and lasts 12-24 hours.

NPH insulin is on the World Health Organization's List of Essential Medicines and was the 221st most prescribed medication in the United States in 2020, with over 2 million prescriptions.

What Is NPH Insulin Used For?

NPH insulin is FDA-approved to improve glycemic control in adult and pediatric patients with:

• Type 1 diabetes mellitus: Often used as the basal (background) insulin component in combination with a rapid-acting mealtime insulin.
• Type 2 diabetes mellitus: Often used as the first insulin added to oral medications when blood sugar is not adequately controlled; may be used alone or with metformin or other oral agents.

Proper blood sugar control with NPH insulin can help prevent the serious long-term complications of diabetes, including kidney disease, blindness, nerve damage, and cardiovascular disease.

Brand Names and Formulations of NPH Insulin

• Humulin N (Eli Lilly): Available as vials (10 mL) and KwikPen (3 mL prefilled pen, 5-count). FDA approved 1982.
• Novolin N (Novo Nordisk): Available as vials (10 mL) and FlexPen (3 mL prefilled pen). FDA approved 1991.
• ReliOn Novolin N (Walmart): Same medication as Novolin N, sold under Walmart's private label for approximately $25/vial OTC. Does not require a prescription in most states.

All formulations are 100 units/mL (U-100). NPH insulin appears as a white, cloudy suspension — not clear like regular insulin. Always check that it appears uniformly cloudy after gentle rolling before injecting.

How Is NPH Insulin Dosed?

Dosing of NPH insulin is highly individualized. The following are general starting guidelines — your doctor will adjust based on your blood glucose results:

• Type 1 diabetes: ADA recommends 0.4 to 1.0 units/kg/day subcutaneously (higher doses may be needed during illness or growth spurts). Usually given twice daily to provide 24-hour basal coverage.
• Type 2 diabetes: Start at 0.1-0.2 units/kg/day subcutaneously, adjusted over days to weeks. Often started once daily at bedtime.

NPH must be injected subcutaneously (under the skin) in the abdomen, thigh, upper arm, or buttocks. It must never be injected into a vein or muscle, and cannot be used in an insulin pump.

How to Prepare and Inject NPH Insulin

1. Check that the insulin looks white and cloudy. Do not use if it looks clear or has lumps.
2. Gently roll the vial between your palms 10 times (do not shake) to resuspend the insulin uniformly. For pens: roll 10 times and invert 10 times.
3. Measure your dose carefully. Small errors in insulin dosing can significantly affect blood sugar.
4. Inject into the prescribed site, rotating to a different spot each time.

How to Store NPH Insulin

• Humulin N: Refrigerate unopened vials. Opened vials: room temperature (up to 86°F/30°C) for up to 31 days. Opened KwikPen: room temperature for up to 14 days.
• Novolin N: Refrigerate unopened vials. Opened vials: room temperature (up to 77°F/25°C) for up to 42 days. Opened FlexPen: room temperature for up to 42 days.

Never freeze NPH insulin — freezing prevents proper resuspension and makes it unsafe to use. Discard if it has been frozen.

If you are having trouble finding your NPH insulin at a pharmacy near you, medfinder can call pharmacies near you to check which ones have it in stock. Visit medfinder.com to get started.